This invention relates generally to standing wave particle beam accelerators, and more particularly to charged particle beam accelerators wherein the standing wave in at least one side coupling cavity can be switched to at least two different asymmetries with respect to the coupling of electromagnetic fields to the two adjacent main cavities, to switch the energy of the particle beam.
Standing wave particle beam accelerators have found wide usage in medical accelerators where the high energy particle beam is employed to generate x-rays. In this application, the output x-ray energy must be stable. It is also desirable that the energy of the particle beam be switchable readily and quickly to provide x-ray beams of different energies to enable different x-ray penetration during medical treatments.
One technique for controlling the beam energy is to vary the rf energy applied to the accelerating cavities. Other implementations have been described in various patents. In U.S. Pat. No. 4,286,192 to Tanabe and Vaguine the energy is controlled by reversing the accelerating fields in one part of the accelerator to decelerate the beam. In U.S. Pat. No. 4,382,208 to Meddaugh et al., the electromagnetic field distribution is changed in the coupling cavity to control the fields applied to the adjacent resonator cavities. U.S. Pat. No. 4,746,839 to Kazusa and Yoneda discloses the use of two coupling cavities which are switched to control the acceleration fields.
It is an object of the present invention to provide a switchable energy side-coupled standing wave particle beam accelerator.
It is another object of the present invention to provide a switchable energy side-coupled cavity standing wave particle beam accelerator which is switchable to provide three levels of output energy with an insubstantial change in frequency and energy spectrum spread.
To achieve the foregoing and other objects of the invention, the particle accelerator includes an input cavity for receiving the charged particles, intermediate accelerating cavities and an output cavity, and a plurality of coupling cavities connecting adjacent pairs of said cavities along the accelerator, at least one of said coupling cavities including means for switching the magnitude of the electromagnetic field coupling to adjacent cavities between a first level and at least two additional levels to provide output energy at least three levels.